Geochemical study on groundwater flow research; Groundwater survey by by dissolved noble gases in groundwater from Tono area

Morikawa, Noritoshi

Noble gas elemental and isotopic compositions in groundwaters may physically vary due to the accumulation of radiogenic component produced in situ and/or from deep flux. As a natural tracer; noble gas can be therefore considered to estimate the groundwater residence time, origin and groundwater flow. In this report, the determination of noble gas contents and isotopic compositions in groundwaters from Tono area was performed in order to evaluate the validity of dissolved noble gas on the groundwater survey. The following results can be reached through the present study: (1)Groundwaters from Tono area contain radiogenic helium, which was released from rocks around the path of groundwater flow and/or flux from the basement granite. In the sedimentary rocks, the amount of accumulated helium in groundwaters increases with depth, whereas such phenomena in fractured granite varies from borehole to borehole. (2)The neon and argon isotopic compositions in groundwaters from Tono area are not distinguishable from those in air-saturated water. (3)Calculation of the two-dimensional helium transport model (Stute et al,, 1992) indicates that the depth profile of dissolved helium concentration in groundwater from DH-12 and DH-13 boreholes are similar to the charactelistics of the discharge and recharge areas, respectively. This is consistent with the hydrogeological model that groundwater around Tono mine is recharged from the north and discharged into the Toki river in southern part (Saegusa et al., 1998).